1. Field of the Invention
The present invention relates generally to an internal combustion engine having a lean NOx catalyst and, more particularly, to an internal combustion engine having a lean NOx catalyst for purifying nitrogen oxide in an exhaust gas by use of a reducer when an exhaust system is especially in an oxygen excessive atmospheric air state, i.e., in a lean state, the lean NOx catalyst is used as a ternary catalyst in a catalyst converter provided in an exhaust system of the internal combustion engine and functioning as an exhaust gas purifying device.
2. Description of the Prior Art
What is known as a catalyst used in the internal combustion engine such as a gasoline direct injection lean-burn engine, a diesel engine and the like, in which the atmospheric air in the exhaust system is in the oxygen excessive state while quantities of hydro carbon and carbon monoxide are small, is a so-called lean NOx catalyst for purifying nitrogen oxide contained in the exhaust gas by using the hydro carbon and the carbon monoxide as reducing agents.
The lean NOx catalyst disclosed in, e.g., Japanese Patent Application Laid-Open Publication No. 6-117225 involves the use of the hydro carbon as the reducing agent. According to the technology disclosed in this Publication, the hydro carbon is partially oxidized to generate active species, and the nitrogen oxide is purified by reducing it by causing reaction between the thus generated active species and the nitrogen oxide.
The hydro carbon as the reducing agent is a main component of a gasoline and a light oil as fuels of the internal combustion engine. Accordingly, as for a supply of the hydro carbon to the lean NOx catalyst, an extra fuel may be supplied into the cylinders separately from other fuels such as the gasoline supplied into the cylinders when in a normal combustion in the internal combustion engine. The extra fuel might cause an incomplete combustion, and hence the exhaust gas is discharged to the exhaust system in a state where the exhaust gas contains a raw gas, i.e., the hydro carbon. Then, according to the above Publication, the extra fuel is supplied by effecting a fuel injection different from the fuel injection normally conducted at an intake-compression stroke. Then, the former normal fuel injection of these injections is called a main injection, and the latter different fuel injection is referred to as an auxiliary injection. The auxiliary injection is executed by changing an injection timing within the same intake-compression stroke other than the injection at the intake-compression stroke based on the main injection, or executed at an exhaust stroke in addition to the main injection. Further, a fuel injection device used for the main injection serves also as a fuel injection device used for the auxiliary injection.
According to this technology, however, since the same fuel injection device performs the main injection and the auxiliary injection as well, a large load must be put on the fuel injection device. Hence, there arises a problem in which a durability of the fuel injection device declines.